Cosmic Ant Nests: How Close Can Stars Be?

If you stare at the sky at night, you will see many thousands of stars. Still, this number is not that bad, because the sun is in a fairly quiet neighborhood about 26,000 light-years from the galactic center.

There are places in our universe where stars are extremely close together, namely in globular clusters and in the centers of galaxies. If you were to look around you in such a star cluster, you would see hundreds of times as many stars.

Globular clusters

Globular clusters are groups of stars that are held together by gravity. In such a group there are hundreds of thousands of stars. About 150 globular clusters are known in our galaxy. This week’s space photo is a stunning shot of globular cluster NGC 1898. This globular cluster is not in the Milky Way, but in the Large Magellanic Cloud, one of the Milky Way’s satellite galaxies.

Globular cluster NGC 1898 (Hubble)

In these globular clusters, stars are packed together like herrings in a barrel. The density is one hundred to one thousand stars per cubic parsec. Converted: a cubic parsec is 3.32 cubic light-years. The density near the Sun is only 0.1 star per cubic parsec.

Four million stars per parsec

Can it be even more extreme? Yes, of course. The ancient globular cluster M15 contains roughly 100,000 stars, most of which are close to the center. The density in the center is four million stars per parsec, but this density decreases rapidly outwards. Even the Hubble Telescope is unable to separate individual stars at the center of this cluster, while the globular cluster is only 37,000 light-years away. Now you may think that there are regular collisions in the center of this globular cluster, but that is not so bad. Scientists suspect that fewer than one in 10,000 stars have fused together in globular clusters.

Globular cluster M15 photographed by Hubble.

Galactic centers

In some galactic centers the density is even higher. This applies, for example, to the galaxy M32, one of the neighbors of the Andromeda galaxy. The density in the center is twenty million stars per cubic parsec. The distance between stars is 0.008 light-years on average, or 500 times the distance from Earth to the sun. That is 12 times the distance from the sun to Pluto.

Fun fact: in M32 stars are so close together that it is never really dark. Scientist Tod Lauer once calculated that the night sky in the center of M32 is comparable in brightness to twilight on Earth.